This invention generally relates to optical amplifiers, and is specifically concerned with a double-pumped Raman amplifier that provides a greater amount of gain per unit length of gain fiber.
Optical amplifiers are well known in the prior art. Such amplifiers are used in optical communication networks both as relay stations for single, long-distance optical circuits, and for boosting signal strength in shorter-distance optical circuits of the type installed around an urban area.
There are generally two types of optical amplifiers, including erbium-doped fiber amplifiers (EDFAs) and Raman-type amplifiers. While both types utilize a pump laser coupled to a gain fiber, the operation of each is quite different. In EDFAs, the gain fiber is doped with erbium or some other rare-earth metal with similar properties. Amplification is accomplished by the excitation of the dopant atoms in the fiber to a high quantum level by the pump laser. An optical signal conducted through such a gain fiber will cause the excited atoms to fall back to a lower quantum level which in turn amplifies the signal through constructive interference. By contrast, Raman-type optical amplifiers utilize conventional transmission optical fiber (i.e., conventional germanium-doped fiber) as the gain fiber. Light from a laser pump is transmitted through such a fiber in the opposite direction as the optical signal being conducted therethrough. The forward-propagating signals achieve gain in the fiber because higher-energy (shorter wavelength) pump photons scatter off the vibrational modes of the optical fiber""s  lattice matrix and coherently add to the lower-energy (longer wavelength) signal photons. While it is possible to achieve amplification by pumping a Raman amplifier in the same direction as the input signal, backward pumping is greatly preferred over forward pumping due to the fact that pump noise is present at a much higher level in the output signal when the pumping is conducted in a forward direction. The maximum gain levels that can be achieved with such Raman amplifiers are typically less than those achievable by EDFAs. However, because Raman amplifiers require no specially doped optical fiber and often include fewer components than EDFAs, Raman amplifiers are often more economical than EDFAs and are often preferred over them.
Because the Raman scattering process in typical transmission fibers is relatively inefficient, several kilometers of fiber are required to achieve high gains at reasonable pump efficiencies. Unfortunately, the use of long fiber lengths on the order of 5 kilometers or more creates several problems that compromise the overall performance of such amplifiers. For example, a type of noise as multi-path interference (MPI) increases when such long lengths of fiber are used as the gain fiber. MPI is caused by double Rayleigh back scattering wherein a fraction of the signal which gets Rayleigh scattered within the fiber is captured by the fiber and propagates in a direction opposite to that of the signal. This scattered light is amplified as it propagates and also undergoes further Rayleigh scattering. Some of the doubly scattered light is captured by the fiber and now co-propagates with the signal causing interference at the receiver. Since the scattered light has been passed through the amplifier twice, it sees twice the gain of the signal. Accordingly, cross talk from double Rayleigh back scattering increases rapidly with increasing gain. Another such problem is that of nonlinear interactions within the amplifier. Such a problem manifests itself in other species of noise known as four wave mixing and self and cross-phase modulation. Such problems increase with not only a long length of gain fiber, but with fiber characteristics which promote Raman amplification within the fiber, such as an increased percentage of germanium dopant, and a small mode-field diameter on the order of 4-5 micrometers.
Hence, there is a need for an improved type of Raman optical amplifier that is capable of achieving higher levels of signal gain with smaller lengths of gain fiber with a same or larger signal to noise ratio. Such an improved Raman amplifier would not only solve the aforementioned noise problems that result from the use of long lengths of gain fiber, but would also make it possible for other optical network components, such as dispersion compensating modules (DCMs) to provide the gain fiber necessary for a high-gain Raman-type amplifier. While DCMs typically include a loop of the type of high germanium, small-mode field diameter fiber that exhibits high degrees of Raman amplification, the fiber loops within such DCMs is only about 3 kilometers, which is less than that typically needed for effective Raman amplification. Accordingly, if a way could be found to achieve a high degree of Raman amplification with only 3 kilometers of gain fiber and no additional noise, such DCMs could be effectively employed not only as dispersion reducing components, but as amplification components as well.
The invention is a Raman optical amplifier that overcomes or at least ameliorates all of the aforementioned shortcomings. To this end, the amplifier of the invention comprises a length of optic Raman gain fiber, and a source of pump light coupled at opposite ends of the fiber for transmitting pump light in opposite directions as an optical signal is conducted through the fiber, thereby providing a double-pass of pump light. The source of pump light may include a single pump light source that is coupled at a downstream end of the gain fiber to transmit pump light in a direction opposite to the optical signal, and a reflector coupled to an opposite end of the gain fiber for reflecting remnant pump light back through the gain fiber in the same direction as the optical signal. Alternatively, the light reflector may be either a mirror coupled to the input end of the gain fiber via a wave division multiplexer, or a fiber Bragg grating written directly into the input end of the fiber. Both the mirror and the fiber Bragg grating may be adjustable to change either the mirror angle or the grating length in order to provide a gain control mechanism for the amplifier. In the case of the fiber Bragg grating, such length adjustment may be accomplished through the use of piezoelectric transducers.
In an alternative embodiment, pump light may simultaneously be provided on both the input and output ends of the gain fiber via a pair of optical pumps, or a single pump in combination with a beam splitter.
In another embodiment of the invention, a bi-directional pumping scheme may be used in combination with a pair of serially connected coils of gain fiber. In such a configuration, a pump light reflector in the form of either a fiber Bragg grating or a pair of mirrors may be optically coupled between the two coils of gain fiber to reflect both the forward and backward propagating pump light for an extra measure of amplification efficiency.
According to an embodiment of the present invention, in order to suppress multiple path interference (MPI) type noise in such a configuration, an optical isolator may be provided between the two mirrors or between two fiber Bragg gratings coupled between the coils of gain fiber. Additionally, other mid-stage components such as gain flattening filters or variable optical attenuators may be coupled between the two gain fiber coils in order to reduce the amount of tilt and ripple in the amplifier output.
In still another embodiment of the invention, a single pump may be coupled to the output end of the amplifier, and a remnant pump loop may be coupled between the two coils. In this embodiment remnant pump light is used to power Raman amplification in the coil nearest the input end of the amplifier.
By providing better absorption of pump light, the optical amplifier of the present invention provides higher levels of gain for shorter lengths of gain fiber. Consequently, the invention advantageously allows the coil of fiber used within dispersion compensating modules to effectively serve the additional function of gain fiber for a Raman amplifier. Also, because only part of the gain is provided by pump light propagating in the same direction as the signal, the requirements for a very low noise pump source are relaxed compared to the case of an amplifier using only forward pumping.